Certain embodiments of the invention provide a polypeptide comprising:(a) an amino acid sequence encoding a first chromophore(b) a first linker(c) an amino acid sequence having at least about 80% sequence identity to an amino acid sequence encoding a troponin C(d) a second linker and(e) an amino acid sequence encoding a second chromophorewherein the first and second chromophores are a Förster resonance energy transfer (FRET) donor-acceptor pair wherein the first chromophore is a donor chromophore and the second chromophore is an acceptor chromophore, or wherein the first chromophore is an acceptor chromophore and the second chromophore is a donor chromophore and wherein the polypeptide is capable of integrating into a sarcomere. Certain embodiments of the invention also provide methods of using such polypeptides.
